Emissions of sulfur oxides from the combustion of sulfur-containing coals is generally considered one of the main causes of the "acid rain" which is damaging lakes, streams and forests, particularly in Northeastern states, and straining relations with Canada. Although these oxides, commonly referred to as SOx, can be scrubbed from the stack gases of utility and industrial boilers, the process is expensive and incomplete. A leading alternative is to substitute for high and medium sulfur coal a solid fuel naturally low in sulfur. Such substitution, however, places a premium price on these low sulfur grades and, conversely, depresses the price of higher sulfur grades, with serious economic consequences to areas dependent upon mining them.
While varying considerably from case to case, transportation from mine to point of use generally adds substantially to the cost of fuel. Although very conscious of the moisture in fuel they buy, the Electric Power Research Institute (EPRI) has reported that, in 1987, utilities spent $1.6 billion just for shipping water. The energy wasted in evaporating this water was equivalent to 10 million tons of bituminous coal.
Much of the country's reserves of low sulfur coal are located in Wyoming, Mont. and adjacent states, a considerable distance from major markets. They are, in general, high in moisture and low in heating value, adding to the expense of shipping energy to where it is needed. Accordingly, inventors and entrepeneurs have given considerable attention in recent years to conversion processes which drive off moisture and improve the heating value of the low sulfur coals and lignites found in these areas, so that their energy can be delivered more economically.
An example is the conversion described in U.S. Pat. No. 4,052,168 (Koppelman) for which a demonstration unit is under construction in Fort Union, Wyo., the solid fuel product being referred to as "K-Fuel". Another example is the "Charfuel" process, assigned to Carbon Fuels Corporation, whose energy product is a non-aqueous slurry of char in liquid products said to be transportable by pipeline. A charfuel demonstration unit will be cited at Glenrock, Wyo.
A common denominator of these conversions is that they produce a distillate by-product of water, heavily contaminated with complex organic compounds (waste water or aqueous waste), which is very expensive to treat, by known means, to the purity required for discharge into public watercourses. At the same time, most of these conversions are net consumers of energy, especially when taken together with the mines and other (off-site) facilities associated with them.
Similarly, other industrial conversions, including thermal and/or oxidative processing of shales, wood by-products and petroleum residues, result in the inadvertent production of aqueous wastes difficult to purify while, at the same time, consuming considerable heat, steam and/or electrical energy.
Primary treatment of dilute wastes of non-hazardous nature usually consists of settling, with or without the addition of floculating agents. Secondary treatment usually comprises bacterial digestion, aerobic or anaerobic. Following this "biological" treatment a concentrated sewage sludge is commonly separated for disposal. After secondary treatment the waste is usually considered suitable, sometimes after chlorination, for discharge into a nearby watercourse, the primary requirement being a low Biological or Chemical Oxygen Demand (B.O.D. or C.O.D.). To achieve a higher effluent standard, a tertiary treatment with activated carbon may be added. Some dilute industrial wastes may skip the biological step. Newer remediation technology accelerates aerobic digestion by substituting oxygen for air, and a type of molecular filtration utilizing porous membranes, known as "reverse osmosis" is achieving success with selected dilute wastes. In general, all of the foregoing treatments are practically limited to wastes having less than about 2 percent combustible organic content.
To dispose of more concentrated and hazardous wastes land-based disposal methods: ponding, landfill and deepwell injection, are commonly used. Landfills usually limit their acceptance of liquid waste to a small fraction of their main commodity, Municipal Solid Waste (MSW). Because of a growing shortage of suitably located land, and environmental opposition, incineration (burning) of solid and concentrated wastes at atmospheric pressure is increasingly preferred.
If the combustible content is less than about 30 percent, supplemental fuel must be fired to achieve stable combustion and a high enough temperature to destroy stable toxic ingredients or intermediates. Above about 30-35 percent combustibles, the combustion yields recoverable energy. Primarily because of a critical shortage of suitable landfills in many areas, such "waste-to energy" boilers are being installed at a rapid rate. Since the latent heat of all of the water in the fuel is lost up the stack they are thermally inefficient.
Waste-fueled boilers, moreover, create environmental problems of their own. There is a rising tide of opposition from nearby residents who fear that atmospheric emission regulations (at present limited to particulate matter) and standards of operation do not adequately protect their health. At least 26 toxic pollutants have been identified in incinerator flue gas. Moreover, the unburned residue may contain hazardous substances and, itself, be difficult to dispose safely.
The EPA plans to publish a list of incinerator air pollutants by November 1988 and propose New Source Performance Standards a year later, a regulatory effort which does not satisfy environmentalists nor congress. Congress, however, was unable to pass in 1988 either the Clean Air Act Amendment or the Resources Conservation and Recovery Act (RCRA). The impasse reflects, at least in part, the difficulty and expense of removing the pollutants from the large volume of flue gas, characteristic of atmospheric pressure combustion.
Combustible content of waste water from fuels conversion is often in the range of 2-15 percent, above the practical range for the usual dilute waste treatment (such as carbon adsorption and reverse osmosis) but below a concentration capable of supporting atmospheric combustion (incineration). In other words, they can be incinerated only at extravagant expense for auxiliary fuel.
Innumerable commercial, industrial and chemical operations, and even solid waste treatments, produce a by-product of aqueous waste. Many of these wastes can be treated economically by conventional methods. Others, particularly those having a combustible content in excess of about 2 percent, are currently treated only with difficulty and excessive expense.
Patent and technical literature describe two additional methods of purifying aqueous wastes by burning combustible impurities from them. Both employ elevated pressures as well as temperatures. The older of these methods, known as Wet Air Oxidation (WAO), is licensed by Zimpro, Inc. For the most part, it is public domain. The second, known as Supercritical Water Oxidation (SCWO), based on U.S. Pat. No. 4,543,190 (Modell et al), is licensed by Modar, Inc. They are more thermally efficient than incineration because the purified water is discharged in liquid state, conserving its latent heat. Also, the flue gas is ordinarily free of pollutants and requires little or no treatment.
WAO oxidizes combustible impurities from aqueous wastes, imposing sufficient pressure to maintain them in liquid phase at temperatures which generally range from 400-650 degrees. SCWO oxidizes the impurities at temperatures and pressures above water's critical of 705.4 degrees and 3200 psi. While both methods are effective, they require expensive equipment, suitable for pressures generally in the range of 2000-4000 psi.